A wide variety of configurations, such as thermoplastic prepregs, yarn, or glass mats (GMT), are known as molding materials comprising continuous reinforcing fiber bundles and thermoplastic resin as the matrices. Such molding materials are characterized in that molding is easy because of a feature of a thermoplastic resin, it is free from a burden of storage unlike a thermosetting resin, toughness of the resulting molded product is high, and its recycle efficiency is excellent. In particular, molding materials that have been processed into the form of pellets can be applied for molding techniques that are excellent in economic efficiency and productivity, such as injection molding or stamping molding. Thus, such materials are useful as industrial materials.
In order to impregnate a thermoplastic resin into a bundle of continuous reinforcing fibers during a process of molding material production, however, such material is problematic in terms of economic efficiency and productivity. Thus, applications thereof are not very extensive at present. For example, it is well-known that impregnation of a resin into a reinforcing fiber bundle becomes more difficult as the melt viscosity of the resin becomes high. In particular, a thermoplastic resin having excellent dynamic properties, such as toughness or ductility, is a polymer, which has higher viscosity than a thermosetting resin and requires higher process temperature. Thus, such material was inadequate in order to produce a molding material easily with good productivity.
When a low-molecular-weight; i.e., low-viscous, thermoplastic resin is used as a matrix resin because of an ease of impregnation, disadvantageously, the dynamic properties of the resulting molded product would be significantly deteriorated. A molding material comprising a composite of a low-molecular-weight thermoplastic polymer and continuous reinforcing fibers and a high-molecular-weight thermoplastic resin in contact therewith is disclosed (e.g., JP Patent Publication (kokai) No. H10-138379 A (1998)).
This molding material involves the use of a low-molecular-weight compound to impregnate into a bundle of continuous reinforcing fibers and a high-molecular-weight compound as a matrix resin. Thus, such molding material is satisfactory in terms of economic efficiency, productivity, and dynamic properties. When this molding material is subjected to injection molding, such material is easily blended with a matrix resin while minimizing damage on the reinforcing fibers at the time of material plasticization during molding. Thus, a molded product that is excellent in fiber dispersion can be produced. Accordingly, the resulting molded product can have longer reinforcing fibers than conventional molded products and such molded product can have satisfactory dynamic properties and excellent appearance quality.
In recent years, however, a fiber-reinforced composite material has drawn much attention, applications thereof becomes more extensive and specialized, a molding material that is excellent in molding performance, handleability, and dynamic properties of the resulting molded product has become desired, and higher economic efficiency and productivity are required at industrial levels. A wide variety of technical development has become required. For example, the impregnation performance of a low-molecular-weight compound is to be enhanced to reduce the operational burden, a molding material having higher thermoresistance is to be suggested, fiber dispersion at the time of molding is to be more improved to further increase the fiber length to improve the dynamic properties, and the surface appearance is to be further improved.
Under such circumstances, disadvantageously, a low-molecular-weight thermoplastic resin undergoes pyrolysis at a processing temperature in the molding process, the resin generates a pyrolysis gas, which in turn contaminates the environment, and a pyrolysis gas becomes a void in the molded product and deteriorates the dynamic properties. Accordingly, development of a molded material that has excellent thermoresistance and that would not cause the environmental contamination around the molding equipment has been awaited.
A fiber-reinforced composite material comprising a reinforcing fiber and a matrix resin is light weight and it is capable of producing excellent strength properties. Also, an arbitrary strength can be realized via regulation of fiber orientation. Thus, such material is extensively used for sports applications, such as golf shafts and fishing poles, aerospace applications, such as aircraft parts and satellite parts, and general industrial applications, such as automobiles and boats and ships, electric/electronic equipment chassises, robot parts, windmills, tanks, bathtubs, and helmets. When producing a fiber-reinforced composite material, a method involving the use of prepreg materials each comprising a reinforcing fiber, which has been impregnated with a matrix resin in advance, as an intermediate substrate to prepare a prepreg laminate is extensively carried out because such method is generally easy to enhance a fiber content and is relatively easily carried out. As a matrix resin to be impregnated into a reinforcing fiber in a prepreg, thermosetting resins, such as unsaturated polyester resin, vinyl ester resin, or epoxy resin, are often used because of ease of impregnation into a fiber bundle. Thermosetting resin becomes an insoluble and infusible polymer having a three-dimensional network structure via curing, such polymer is difficult to recycle, and disposal thereof becomes more serious.
A variety of resins, such as polyethylene, polyester, polyamide, or polycarbonate, are used as thermoplastic matrix resins for prepregs. In the case of aerospace applications or the like that require high performance, polyether ether ketone, polyetherimide, polyphenylene sulfide, or the like that is excellent in terms of thermoresistance, chemical resistance, and mechanical properties is preferably used. Use of polyarylene sulfides, such as polyphenylene sulfide, is particularly preferable.
However, the molecular weight of such thermoplastic resin, prepreg, is higher than that of a thermosetting resin during the process of production in which a fiber bundle is impregnated with a matrix resin. Thus, processing of such thermoplastic resin needs to be carried out at a high temperature and a high pressure, and production of prepregs having a high fiber content is difficult. Also, the produced prepregs are often not impregnated, and mechanical properties are not satisfactory.
In order to overcome such drawbacks, a method for preparing prepregs wherein a slurry of polyarylene sulfides is prepared in a dispersion medium to facilitate impregnation thereof into a glass fiber mat (e.g., JP Patent Publication (kokai) No. H5-39371 A (1993)) and a method of preparing a sheet of a polyarylene sulfide having a relatively low molecular weight, and laminating the same with a fiber substrate to prepare a laminate without a prepreg (e.g., JP Patent Publication (kokai) No. H9-25346 A (1997)) are known. The former method, however, requires equipment and time to remove a dispersion medium. Further, it is difficult to completely remove the dispersion medium, and mechanical properties would be disadvantageously unsatisfactory because of voids resulting from evaporation of the dispersion medium at the time of lamination. Also, the latter method requires molding to be carried out at a high temperature and a high pressure, and mechanical properties would be disadvantageously unsatisfactory because of insufficient impregnation.
A fiber-reinforced composite material comprising a bundle of continuous reinforcing fibers and a matrix resin has light weight and excellent dynamic properties. Such material is extensively used for sports applications, aerospace applications, and general industrial applications. In particular, a composite material comprising a carbon fiber as a reinforcing fiber (CFRP) has specific strength and specific rigidity that are superior to those of metal materials, and the amount thereof used is increasing mainly for aerospace applications. To date, a thermosetting resin has been preferably used as a matrix resin because of its good impregnation into a reinforcing fiber bundle. A thermoplastic resin has a high molecular weight and higher viscosity than that of a thermosetting resin, and processing thereof needs to be carried out at a higher processing temperature. Accordingly, a thermoplastic resin was not suitable for producing a fiber-reinforced molding substrate easily with good productivity.
In recent years, however, a composite material comprising a thermoplastic resin as a matrix resin is effective in order to shorten the molding time, the molded product resulting therefrom is advantageous for recycle, and post-processability, such as heat adhesion or heat correction, is excellent. Thus, such composite material has drawn attention in various applications. Among thermoplastic resins, polyarylene sulfide has high elastic modulus, high thermoresistance, and excellent fluidity. From the viewpoint of improvement in dynamic properties of molded products, such composite material can be preferably used as a fiber-reinforced composite material. Thus, a method for producing a fiber-reinforced molding substrate comprising a bundle of continuous reinforcing fibers and polyarylene sulfide in a more cost-effective and productive manner has been awaited.
As a method for producing a fiber-reinforced molding substrate comprising a bundle of continuous reinforcing fibers and a thermoplastic resin, for example, a method comprising placing crystalline thermoplastic resin films on the surface and the opposite surface of a sheet-like reinforcing fiber bundle, and applying a pressure at 5 to 30 kg/cm2 (about 0.5 to 3 MPa) at a temperature that is 150° C. higher than a resin melting point to impregnate the thermoplastic resin into the reinforcing fiber bundle is proposed (see JP Patent Publication (kokai) No. H8-118489 A (1996)). In this method, however, impregnation of thermoplastic resin needs to be carried out at harsh temperature. This disadvantageously causes resin pyrolysis, and properties of the molded product cannot be brought to a satisfactory level. Thus, it is difficult to produce a molding substrate in a cost-effective and productive manner.
In order to easily impregnate thermoplastic resin into a bundle of continuous reinforcing fibers, a method for producing a molding material comprising first impregnating a low-molecular-weight thermoplastic resin into fiber and integrating the fiber with a high-molecular-weight thermoplastic resin is proposed (see JP Patent Publication (kokai) No. H10-138379 A (1998)). According to this method, impregnation performance can be satisfactory with the use of a low-molecular-weight thermoplastic resin. However, handleability of the molding material is disadvantageously insufficient, and it is difficult to bring the properties of the molded product to a satisfactory level.
Thus, a method for producing a fiber-reinforced molding substrate comprising polyarylene sulfide impregnated into a bundle of continuous reinforcing fibers in a simple and productive manner has not yet been satisfactorily proposed.